In for instance pneumatic systems, which to a great extent are used for mechanizing and automating heavy as well as light working conditions, oil mist is added to the compressed air via a so called oil mist lubrication aggregate for the pneumatic system to function. Since, however, this has a detrimental effect on the working environment and is dangerous to the health, it has for many years been an aim to cease using the admixture of oil mist.
With oil-free compressed air great demands must of necessity be put upon the pneumatic components. These great demands have comparatively well been met as to valve components. However, for instance concerning cylinders and pistons the problems to achieve required life and sealing against cylinder pipe and piston rod have not been overcome. The cylinder pipe is today among other things for corrosion reasons practically always made of aluminum--like many other pneumatic components, and the demands upon the inner surface of the aluminum pipe are satisfactory resistance to wear and low friction even when there is lack of lubricant. Moreover, a surface coating must be well adhering. In attempts to meet these demands there are method based on anodic treatment and Teflon (trademark) inpregnation. A disadvantage with these methods is that they are difficult to carry out in long pipes and this fact in combination with the comparatively large surfaces existing in pneumatic cylinders means, that the product becomes comparatively very expensive. As to piston rods, which usually are hard chromium plated, they must have a very fine surface but nevertheless great surface wear is obtained. Moreover, chromium is porous and often has crack formations and therefore action of rust and damages are obtained resulting in lowered life. In the dry state, i.e. without lubrication, the anodically treated surface and the chromium coated surface files off the sealings, usually sealings of rubber, and oxides of aluminum and chromium particles in-baked in sealing, piston guide and bushing wear on the pipe and the piston rod and deteriorate the surface. An accelerated wear course is obtained resulting in unsatisfactory life and sealing. Due to this one has been restricted to use a continuous lubrication with oil mist, in spite of the above mentioned disadvantages, often in combination with an initial lubrication with grease.
In hydraulic systems, in which the above mentioned problems with lubrication do not exist, one has, however, an essentially higher pressure and stress on the sealing surfaces. For obtaining a sealing satisfactory from the wear point of view the formation and maintenance of an oil film between sealing and co-operating surfaces is, however, required as well as a surface having high surface fineness. However, for practical and economic reasons the surfaces are too rough for obtaining quite a satisfactory result in this connection. The result of this is among other things that also with finest conceivable surfaces the sealings must be exchanged owing to wear down. When using for instance chromium-plated piston rods there are furthermore the above mentioned problems in view of the fact that chromium is porous and often has crack formations.